A homopolar generator is a unique electrical generator, sometimes referred to as a Faraday disk after Michael Faraday, who developed the basic device in 1831. The homopolar generator can convert rotational energy into direct current by rotating an electrically conductive disc in a plane perpendicular to a magnetic field. Faraday's law of electromagnetic induction and/or Lorentz's force law can be utilized to explain the operation of the homopolar generator. The radial movement of the electrons in the disc in the presence of the magnetic field produces a charge separation between the center of the disc and its rim, and if the circuit is completed between the disk center and rim, an electric current will be produced when the disc is rotated.
Versions of the homopolar generator have been used to supply currents up to 2 million amperes, but practical use has been limited due primarily to the low voltage output and high I2R losses that can arise from imperfect electrical brush connections to connect external circuits.
Conventional battery chargers and alternators are typically designed for charging several battery cells in series, with charging currents limited by the additive internal resistance of each cell in the series. Charging cells of a battery in series tends to require extended charging periods. For example, a typical multi-cell 10 ampere-hour battery requires roughly 15 hours to reach full charge from a fully discharged condition with a 1 ampere charger.
Electric cars and other electric vehicles utilize battery packs having multiple rechargeable cells that are connected in series to provide adequate voltages for driving electric motors. One of the barriers for commercial success of the all electric car, however, is the long associated battery charging times. Part of the issue that can contribute to the long charging time is the internal series resistance that limits the amount of charging current that can flow through the battery. Some designers and manufactures have proposed systems to swap out the entire battery each time it is discharged to address the charging time issue.